Colleges and universities, long considered major building blocks of social opportunity in the United States, have significantly decreased race-based affirmative action in the past dozen years, according to a UC Davis sociology study.

On average, U.S. public and private institutions of higher education showed only a slight increase in affirmative action through the end of the 1980s and a more substantial decline since, say Eric Grodsky, an assistant professor of sociology, and graduate student Demetra Kalogrides.

Affirmative action policies give preference to a historically disadvantaged class of students over otherwise more advantaged students based on attributes other than demonstrated merit.

In 1996, California voters led the nation to reject affirmative action by barring race- and gender-based preference in all state programs, including public university admissions.

The most prestigious universities are still more likely to claim preferences for minority students than are higher education institutions with lower test scores and fees.

Grodsky and Kalogrides speculate that one reason the nation's premier universities sponsor students from disadvantaged backgrounds is a strong commitment to noblesse oblige -- the obligation of responsible behavior by those with high rank in a society.

The two sociologists used data from an annual survey of colleges and universities collected by the College Board between 1986 and 2003.

They found that more than half of the comprehensive colleges and universities in the United States claim to engage in affirmative action. But the degree to which the policies are used varies greatly.

"How far alternative policies have gone in preserving the access of minority students to higher education remains an open question," Grodsky and Kalogrides conclude, suggesting that the question is increasingly urgent.

"If schools are acting in ways that conform to their stated preferences, there is reason to believe that the progress we have made in improving access to competitive postsecondary institutions in eroding."